Question

What do the following genetic symbols stand for: AA, Aa.

Answer 1

Various different symbols are used to represent genes or alleles (alternative forms of the same genes). Upper case alphabets are used for dominant alleles and lower case alphabets are used for recessive alleles.

Complete answer:
Genetic symbols are defined as the set of symbols that are used to represent various genes and include the protocol for nomenclature of genes. An international committee in 1957 published recommendations for genetic symbols and gene nomenclature (the scientific naming of genes).

The symbols used for representation of the genes are either first one or two letters of the name of the genes. For dominant characters, upper case letters are used while lower case letters are used for recessive characters.
For Example: If $R$ is the dominant gene for round form of seed, $r$ will be the recessive gene for wrinkled form of seed.

The genetic symbol $A$ given in question represents the dominant gene and $a$ represents the recessive gene. The genetic symbol $AA$ stands for homozygous dominant because both the genes are dominant and identical. The genetic symbols Aa stands for heterozygous dominant because $A$ is a dominant gene and $a$ is a recessive gene and both the genes are different.

Hence, the correct answer is option (B).

Additional information:
- Homozygous: It refers to the state where both the genes are identical.
Example: $TT$ or tt for tallness.
- Heterozygous: It refers to the state where both the genes are different.
Example: Tt for tallness.
- Dominant: An allele which expresses itself when present in homozygous or heterozygous conditions.
Example: $TT$ (homozygous) and $Tt$ (heterozygous) for tallness, $tt$ (homozygous) for dwarfness.
- Recessive: An allele which expresses itself when present in homozygous condition but remains suppressed in heterozygous condition.
Example: $t$ or dwarf.

Note: From the above discussion it is clear that the genetic symbol $AA$ stands for homozygous dominant and $Aa$ stands for heterozygous dominant. $A$ is the dominant gene and can express itself independently whereas $a$ is recessive gene and cannot express itself in heterozygous condition.